A separator of alkaline secondary batteries is required (1) to hold an alkaline electrolyte, (2) to separate a positive electrode and a negative electrode, (3) to withstand winding tension, (4) to cause no shortcircuit between positive and negative electrodes, (5) to allow ions and water to migrate, and (6) to allow gas produced to permeate.
Conventionally used separators of alkaline secondary batteries include polyamide nonwoven fabric and polyolefin nonwoven fabric having been made hydrophilic by a chemical treatment, grafting or application of a surface active agent (see JP-A-58-94752, JP-A-61-78053, JP-A-64-86445, JP-A-2291665, and JP-A-4-167355, the term "JP-A" as used herein means an "unexamined published Japanese patent application").
Polyamide nonwoven fabric, though excellent in hydrophilic properties, undergoes hydrolysis in an alkaline electrolyte, and the hydrolysis product reduces the capacity retention during storage as pointed in H. W. Lim, et al., Proceedings of the 27th Power Sources Conference, pp. 83-85 (1976).
Polyolefin nonwoven fabric endowed with hydrophilic properties by a chemical treatment or grafting has become brittle due to the treatment as reported in Mori, et al., KOBUNSHI RONBUNSHU, Vol. 48, No. 1, pp. 1-9 (January 1991). As a result, shortcircuits tend to occur during electrode winding or on charging or discharging. Besides, an additional step is required for disposal of the waste treating liquid.
Polyolefin nonwoven fabric having been rendered hydrophilic with a surface active agent has disadvantages that the applied surface active agent is easily dissolved in the electrolyte, resulting in a lowering of capacity retention during storage as discussed in JP-A-64-57568 and that the internal resistance of the battery tends to increase with lowering of capability of holding the electrolyte, resulting in a discharge voltage drop or a lowering of the utilization of the active material.